1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 35

3. Router-Based Congestion Control Solution3

4. Congestion Control Basics: problem, terminology, approaches, metrics
Solutions
Router-based: queuing disciplines
Host-based: TCP congestion control
Congestion avoidance
DECbit
RED gateways
Quality of service

5. Router Solutions: Queuing Disciplines
Router defines policies on each outgoing link
Allocates buffer space:
Which packets are discarded?
Allocates bandwidth:
Which packets are transmitted?
Affects packet latency:
When are packets transmitted?

6. More Fairness Choices First In, First Out (FIFO) Fairness for latency
Minimizes per-packet delay
Bandwidth not considered (not good for congestion)
Fair queuing
Fairness for bandwidth
Provides equal bandwidths (possibly weighted)
Delay not considered

7. Fair Queuing Logical round-robin on bits Idea
Equal-length packets: round-robin on packets
Variable-length packets ?
Idea
Let Si denote accumulated service for flow i
Serve the flow with lowest accumulated service
On serving a packet of length P from flow i, update
Si = Si + P

8. Fair Queuing Example A B C SA SB SC 10 10 20 10 20 15 20 15 20 35 3510 10 20 10 20 15 20 15 20 35 35 20 35 30 20 15

9. Fair Queuing Example Compare Si or Si + P ? A B C SA SB SC 10 10 15 2010 10 15 20 15 20 15 20 30 15 35 30 15 35 30 20 15
Another detail: update counter at start or end of transmission ?

10. Why is the suggested approach not quite adequate?
Fair Queuing
Why is the suggested approach not quite adequate?
Flows can “save up” credit
No transmission for long time (call it T)
Burst uses all bandwidth for up to time
T x flow’s share of link

11. Fair Queuing How might we fix this problem?
Don’t allow inactive flows to retain service rates below that of any active flow
i.e. after updating some flow’s Si
For each flow j with no packets in its queue
Set Sj to the minimum Sk for all active flows k
(or 0 if no flows are active)

12. Fair Queuing Example A B C SA SB SC 10 10 20 15 20 20 35 30 35 10 1010 10 20 15 20 20 35 30 35 20 15

13. Weighted Fair Queuing Extend fair queuing
Notion of importance for each flow
Implement as weight Wi for flow i
Update accumulated service with P/Wi

14. Weighted Fair Queuing Example15 10 10
B (2) 20 10 10 10 15 20 10 10 10 20
C (1) 20 15 SA SB SC 10 10 10 15 20 10 15 20 15 20 15 20 35

15. Weighted Fair Queuing
What makes up a flow for fair queuing in the Internet ?
Too many resources to have separate queues/variables for host-to-host flows
Scale down number of flows
Typically just based on inputs
e.g. share outgoing STS-12 between incoming ISP’s

16. Fair Queuing in the Internet10 10 10 A B 10 10 10 10 10 10 10 10 10
STS-12 C SA SB SC SD 10 10 10 20 10 20 10 20 10 10 10 D
Service-Level Agreements (SLAs) for
STS-3 (155Mbps)
STS-4